1. Technical Field of the Invention
The present invention relates generally to starter motors for starting engines. More particularly, the invention relates to a starter motor which includes a seal plate that hermetically seals the open end of a bearing box formed in an end frame of the starter motor.
2. Description of the Related Art
Japanese Patent No. 3575359, an English equivalent of which is U.S. Pat. No. 6,570,283 B2, discloses a starter motor that generates torque for starting an engine of a motor vehicle.
The starter motor includes, as shown in FIG. 4, a cylindrical bearing box 110 formed at the radial center of a discoid end frame 100. A ball bearing 120 is disposed in the bearing box 110 to rotatably support a rear end portion of an armature shaft 130.
Further, from the open end of the bearing box 110, a cup-shaped bearing cover 140 is inserted in the bearing box 110 to cover the ball bearing 120, thereby sealing the ball bearing 120 within the bearing box 110. The bearing cover 140 is made of rubber or soft resin. The bearing cover 140 has a plurality of annular projections 141 formed on the outer periphery of a cylindrical side wall of the bearing cover 140. The annular projections 141 are press-fit, with a small pressure, to the inner surface of the cylindrical side wall of the bearing box 110.
In the above starter motor, since the bearing cover 140 is made of rubber or soft resin, it is difficult to control the dimension of the bearing cover 140. Moreover, with the annular projections 141, the shape of the bearing cover 140 is complicated, resulting in an increase in the manufacturing cost of the starter motor.
Furthermore, since the bearing cover 140 is press-fit into the bearing box 110 only with a small pressure, the sealing performance of the bearing cover 140 may be lowered by heat generated in the starter motor (e.g., heat generated by brushes of the starter motor) and/or the influence of high temperature inside an engine compartment in which the starter motor is installed. As a result, in the worst case, the bearing cover 140 may be detached from the bearing box 110 by vibration of the vehicle during running.
In addition, to reliably block foreign matter, such as dust or water, from reaching the ball bearing 120, it is necessary to provide a sufficient number of the annular projections 141 in alignment with each in the axial direction of the bearing cover 140. However, this will increase the axial length of the bearing cover 140, and thus will increase the axial length of the bearing box 110 and that of the entire starter motor.